Using screws, build 2 frames with the 8' long pieces
overlapping the 2'-9" long pieces. (See diagram)

You want the frames to be 3'x8' outside measurement.

click on the photo to see full
details

Screw down the 3'x8' floor plywood to one of the frames.
Square the frame to the plywood. The plywood should be
flush to the outside of the frame all the way around.

Flip this assembly over and attach the legs to the inside of
the frame-with-floor and the second frame assembly. The
outside of both frame assemblies should wrap around the
outside of the legs.

Attach wheels at this time if you want to.

Flip this assembly over so that it is now standing upright
on the legs.

Attach the plywood walls to the outside of the 2x4 frame.
Make sure the bottom edge of the walls is flush to the
bottom of the 2x4 frame all the way around. Also make sure
the 8' long side walls are flush to the frame at each end.
This will give you a nice, tight corner. The 3'-1" end walls
will overlap the 8' side walls.

Install the corner nailer's to secure the corners of the
walls.

click on the photo to see full
details

If you want to install a Plexiglas window:

Cut the height and width of the window opening in the wall
3" smaller than the piece of Plexiglas.

Pre-drill the Plexiglas to accept the mounting screws.
(Wear safety glasses when working with the Plexiglas)

Drill the holes at least 3/4" in from edge of Plexiglas or
it will crack.

Drill the holes a larger diameter than the screw threads.

When you drill the holes, lay the Plexiglas on a piece of
scrap wood and drill through slowly or it will crack. Hold
the Plexiglas down while drilling, it will try to ride up
the drill bit and will crack.

Attach the predrilled window to the inside of the wall.
Use a screw driver (not a power screwdriver) and only "snug"
the screws down. If you put too much pressure on the screws
it will crack the Plexiglas.